Elsevier

Neuroscience

Volume 192, 29 September 2011, Pages 11-19
Neuroscience

Cellular and Molecular Neuroscience
Research Paper
Nonspecific effects of the gap junction blocker mefloquine on fast hippocampal network oscillations in the adult rat in vitro

https://doi.org/10.1016/j.neuroscience.2011.07.015Get rights and content

Abstract

It has been suggested that gap junctions are involved in the synchronization during high frequency oscillations as observed during sharp wave-ripple complexes (SPW-Rs) and during recurrent epileptiform discharges (REDs). Ripple oscillations during SPW-Rs, possibly involved in memory replay and memory consolidation, reach frequencies of up to 200 Hz while ripple oscillations during REDs display frequencies up to 500 Hz. These fast oscillations may be synchronized by intercellular interactions through gap junctions. In area CA3, connexin 36 (Cx36) proteins are present and potentially sensitive to mefloquine. Here, we used hippocampal slices of adult rats to investigate the effects of mefloquine, which blocks Cx36, Cx43 and Cx50 gap junctions on both SPW-Rs and REDs. SPW-Rs were induced by high frequency stimulation in the CA3 region while REDs were recorded in the presence of the GABAA receptor blocker bicuculline (5 μM). Both, SPW-Rs and REDs were blocked by the gap junction blocker carbenoxolone. Mefloquine (50 μM), which did not affect stimulus-induced responses in area CA3, neither changed SPW-Rs nor superimposed ripple oscillations. During REDs, 25 and 50 μM mefloquine exerted only minor effects on the expression of REDs but significantly reduced the amplitude of superimposed ripples by ∼17 and ∼54%, respectively. Intracellular recordings of CA3 pyramidal cells revealed that mefloquine did not change their resting membrane potential and input resistance but significantly increased the afterhyperpolarization following evoked action potentials (APs) resulting in reduced probability of AP firing during depolarizing current injection. Similarly, mefloquine caused a reduction in AP generation during REDs. Together, our data suggest that mefloquine depressed RED-related ripple oscillations by reducing high frequency discharges and not necessarily by blocking electrical coupling.

Highlights

▶Mefloquine did not affect ripple oscillations during hippocampal sharp wave-ripples. ▶Mefloquine reduced ripple amplitude during epileptiform discharges. ▶Mefloquine increased afterhyperpolarization following evoked APs and reduced neuronal excitability in CA3 pyramidal cells.

Section snippets

Slice preparation

Animal procedures were performed in accordance with the guidelines of the European Communities Council and approved by the regional authority (LaGeSO Berlin: T0068/02). Wistar rats (aged 6–8 weeks, ∼200 g) of either sex were decapitated under deep ether anesthesia. Horizontal hippocampal slices (400 μm/at bregma −4.7 to −7.3 mm) were prepared at an angle of ∼12 degree in the fronto–occipital direction (with the frontal portion up) using a vibratome (752 M Vibroslice, Campden Instruments,

Effects of mefloquine on sharp wave-ripple complexes

In order to test for effects of mefloquine on SPW-Rs, we induced such events by recurrent high frequency stimulation (HFS) applied to stratum radiatum (SR) in the proximal CA1 region, which induced two population spikes (PSs) in area CA3. During such stimulation the first PS is due to antidromic propagation of APs along CA3 axon fibers (Schaffer collaterals), and the second is due to generation of excitatory postsynaptic potentials (EPSPs) generated by recurrent axon collaterals within area CA3

Discussion

Our findings indicate that mefloquine significantly reduced ripple oscillations during REDs, which were recorded in the absence of GABAergic transmission while ripple oscillations during SPW-Rs were not significantly changed. In CA3 pyramidal cells, 50 μM mefloquine caused a reduction in AP firing during positive current-injection and significantly increased the amplitude of the AHP following APs while neither the resting membrane potential nor the input resistance was changed. Finally, the

Conclusion

Based on our data, we cannot exclude that synchronization during REDs might partly be mediated by Cx36-gap junctions but the present data rather indicate that mefloquine is not specific enough to prove that point.

Acknowledgments

We are grateful to Drs. H. J. Gabriel and H. Siegmund for excellent technical support and development of data analysis tools. This research was supported by the Excellence cluster NeuroCure, by DFG grant he 1173/17-1, GRK 1123 “Cellular mechanisms of learning and memory consolidation” and by a grant from the Hertie Foundation. RuH is a recipient of a grant from the DAAD and the HEC of Pakistan.

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